1. Field of the Invention
The present invention relates to antenna apparatus having antenna elements for transmitting and receiving radio waves to and from a satellite, which antenna apparatus can advantageously be used as mounted on a vehicle for example.
2. Description of the Related Art
In recent years, mobile units have been increasingly computerized with information technology equipment for, for example, allowing drivers or passengers to enjoy watching television by receiving a ground wave as well as to obtain various types of information by accessing Internet through a mobile telephone or the like. To meet the need for further computerization of mobile units, research and development is being made to realize antenna apparatus for use on vehicles which is capable of tracking a satellite for transmitting and receiving radio waves to and from the satellite. Examples of known such antenna apparatus capable of tracking a satellite include an array antenna to perform mechanical beam-scanning, and an array antenna to perform electrical beam-scanning. Specifically, the mechanical beam-scanning array antenna mechanically changes the beam direction of the antenna to track a satellite automatically, thereby ensuring continuous communication with the satellite. A representative of such beam-scanning array antennas is a microstrip Yagi array antenna. On the other hand, the electrical beam-scanning array antenna comprises a plurality of circular antenna elements disposed on a planar substrate for example and is capable of automatically making the beam direction coincide with a satellite direction by electrically controlling the phases of respective antenna elements.
Microstrip array antennas of the mechanical beam-scanning type are usually a narrow band. In applying such a microstrip antenna to antenna apparatus for use on vehicles it is required that the microstrip antenna be adapted for a broader band because it is constructed to realize the functions of transmitting and receiving radio waves both. However, the manufacture of such a microstrip antenna adapted for a broad band is difficult. The microstrip antenna of the mechanical beam-scanning type has many other inconveniences in the application to the antenna apparatus for use on vehicles; for example, the size of its housing will be doubled or more if the transmitting section and the receiving section are separated and, hence, the influence of wind becomes more serious. On the other hand, array antennas of the electrical beam-scanning type involve a cost problem in practical use as antenna apparatus for use on vehicles.
Antennas for use on vehicles primarily for satellite communications at mobile stations are required to improve their antenna gain for a larger data transmission capacity besides other requirement for a low profile, small-sized and light-weight configuration; for example, Engineering Test Satellite VIII (ETS-VIII), the development of which has started since 1998 for the purpose of developing the technology required to realize mobile-satellite communications through mobile terminals and mobile multimedia satellite broadcasting, requires a gain of 12 dBi or more as an objective capability of on-vehicle antennas adapted primarily for satellite communications at mobile stations.
Accordingly, it is a primary object of the present invention to provide antenna apparatus capable of obtaining a high antenna gain with a reduced coupling between transmitting antennas and receiving antennas notwithstanding its configuration made compact and less susceptible to wind and the like.